1. Field of the Invention
This invention relates to a method and apparatus for automatically measuring blood pressure by a noninvasive method. More particularly, the present invention relates to an automatic blood pressure measurement method and apparatus for shortening a measurement process which includes automatically halting cuff pressurization near the systolic blood pressure of a patient, allowing cuff pressure to diminish, determining the systolic blood pressure and then measuring the diastolic blood pressure
2. Description of the Prior Art
In the conventional automatic sphygmomanometer which operates on the basis of the indirect ausculcation method, the pressure applied to a cuff affixed to a limb of a patient is controlled by presetting a pressurization cut-off point (e.g., 150-200 mmHg) high enough to cover the systolic blood pressure of a typical patient, and pressurizing the cuff until this preset value is attained. Accordingly, if we assume that the rate at which pressure is released from the cuff is constant, the time needed to measure blood pressure is prolonged by an amount corresponding to pressurization of the cuff over and above the systolic blood pressure of the patient. Moreover, for a constant pressure-release rate of 2-3 mmHg/sec, several heartbeats in the vicinity of the systolic blood pressure are all that are required when determining such blood pressure. Pressurizing the cuff beyond what is needed to detect these heartbeats requires time that is unnecessary for measurement for all practical purposes. This not only causes the patient discomfort but is also detrimental to achieving rapid measurement for ascertaining a short-term fluctuation in blood pressure.
The blood pressure measurement process with the conventional automatic sphygmomanometer of the above-described type comprises initially pressurizing the cuff to the set value (e.g., 150-200 mmHg) and then depressurizing the cuff at a constant rate (e.g., 2-3 mmHg/sec) of pressure release. Thus, determination of the systolic and diastolic blood pressures is performed by a continuous measurement spanning the interval between these two points. The time required for blood pressure measurement is therefore dependent upon the blood pressure value (pulse pressure) of the patient, assuming that the cuff venting rate is constant. Since the flow of blood during the aforementioned interval is almost completely cut off by occlusion, a state of blood congestion continues for the duration and often results in an erroneous determination of diastolic blood pressure. Furthermore, to perform a meaningful measurement, only a few heartbeats in the vicinity of the systolic and diastolic blood pressures are needed. Measurement conducted at a regulated venting rate during other intervals requires time which is wasted for practical purposes. And as mentioned above, the longer the time needed for measurement, the greater the discomfort experienced by the patient and the more difficult it is to realize high-speed measurement for ascertaining rapid fluctuations in blood pressure.
The aforementioned automatic sphygmomanometer according to the prior art employs a diaphragm-type pump or piston-type pump as the cuff pressurizing means. Noise emitted by the pressurizing means whenever a measurement is taken is a cause of stress not only for the patient but for those in the vicinity. Moreover, since a patient undergoing long-term monitoring of his or her blood pressure will tend to be awakened at night by the noise from the pressurizing means, it is difficult to ascertain the dynamics of the patient's blood pressure during sleep.